Silver halide emulsions have always been used in photographic light-sensitive materials. The preparation of silver halide grains comprises two conventional steps, a nucleation step and a growth step.
For the nucleation step, there are various conventional processes. The single-jet process comprises adding a silver halide aqueous solution into a stirred reactor containing a colloid, usually gelatin, and a halide aqueous solution. In the double-jet processes, the silver salt and halide solutions are added simultaneously or alternately from two separate sources into a stirred reactor containing the colloid. In both cases, growth is generally obtained by a double-jet precipitation.
In the conventional processes, it may be difficult to correlate the number of nuclei obtained with the final number of grains, especially because of the Ostwald ripening which favors the growth of large grains rather than small grains, due to their different solubility. For a given number of nuclei, the number of grains after growth will usually be less than the number of nuclei.
There is a third process which comprises carrying out the nucleation step inside a first reactor by simultaneously adding the silver salt solution, the halide solution and the colloid solution, and in carrying out the growth step in a second reactor by adding a silver salt solution and one or more halide solutions.
In the prior art, the preparation process for ultra-thin silver halide emulsions is well known and comprises using, for example, growth inhibitors. The Patent Application EP 431 584 describes a preparation process for an ultra-thin grain emulsion with grain size less than or equal to 0.05 .mu.m. The preparation process for such an emulsion includes adding a silver salt solution and a halide solution into a stirred reactor containing at least one high molecular weight compound and a compound capable of adsorbing on the silver halide grains, given that the high molecular weight compound and the growth inhibitor both have a retarding effect.
U.S. Pat. No. 5,254,454 describes a preparation process of silver halide grains for photographic emulsion, wherein the nucleation step is carried out in a strongly stirred mixer (10,000 rpm) by adding a silver salt solution, a halide solution and a colloid solution. According to U.S. Pat. No. 5,254,454, a first emulsion is thus obtained which contains fine silver halide grains (size less than or equal to 0.01 .mu.m). This first emulsion is then transferred to a reactor in which the pAg is modified. The modified emulsion is then transferred to a second reactor containing a second emulsion composed of nuclei. The nuclei contained in the second reactor will encourage, after dissolution, the growth of the fine grains of the first emulsion.
Considering the variety of silver halide photographic emulsions used in photographic materials, it would be highly preferable to have a preparation process, using a single nucleation step, of emulsions that exhibit identical or different silver halide grain size or grain size diversity.